This invention relates to an air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to a secondary air control device for controlling a shot air valve which is adapted to allow temporary supply of atmospheric air into the intake pipe of the engine to reduce the concentration of a too rich mixture in the intake pipe when there occurs a sudden increase in the negative pressure in the intake pipe.
An air/fuel ratio control system for performing the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having an intake pipe has already been proposed by the assignee of the present application, which comprises means for detecting the concentration of an exhaust gas ingredient emitted from the engine, fuel quantity adjusting means for producing the air/fuel mixture being supplied to the engine, and an electrical circuit operatively connecting the concentration detecting means with the fuel quantity adjusting means in a manner effecting feedback control operation to control the air/fuel ratio of the mixture to a predetermined value in response to an output signal produced by the concentration detecting means.
On the other hand, a shot air valve is generally used, whhich is adapted to allow temporary supply of atmospheric air into the intake pipe of the engine upon a sudden increase in the suction negative pressure in the intake pipe which is caused, for instance, by a sudden change in the throttle valve position from its full opening position to its full closing position, to make leaner the mixture being supplied to the engine, which is then too rich due to the suddenly increased negative pressure, to thereby prevent explosive combustion of unburned fuel within the exhaust system of the engine.
However, even at the start of the engine, this shot air valve is actuated due to a sudden increase in the suction negative pressure in the intake pipe at the start of the engine so that the suction mixture is temporarily diluted, which can cause misfire in engine cylinders.